Mirror certain part of a mesh
Symmetrize or Mirror

While in edit mode, you have two main options:

1) run the Symmetrize operator:


  The Symmetrize tool is a quick way to make a mesh symmetrical. Symmetrize works by cutting the mesh at the pivot point of the object, and mirroring over the geometry in the specified axis, and merges the two halves together (if they are connected)




2) or the Mirror operator:


  The mirror tool mirrors a selection across a selected axis. [..]
  The mirror tool in Edit mode is similar to Mirroring in Object mode. It is exactly equivalent to scaling by -1 vertices, edges or faces around one chosen pivot point and in the direction of one chosen axis, only it is faster/handier.




Both quotes comes from the manual.

The difference between the two methods, apart from the little differences in the usage, is that while Symmetrize wil keep your original selection in place, Mirror will instead "move" that selection on the other side.

Note: you'll probably need to recalculate normals (Crtl+N) after a mirroring operation.